Guide to TCP/IP, Third Edition : Guide to TCP/IP, Third Edition Chapter 8:
The Dynamic Host Configuration Protocol
Objectives : IP Addressing and Related Topics 2 Objectives Understand the basic services DHCP offers to its clients and explain its background
Understand the specifics of IP address management using DHCP
Explain the DHCP Discovery, renewal, and release processes
Understand the basic DHCP packet structure and types of DHCP messages in use
Objective (continued) : IP Addressing and Related Topics 3 Objective (continued) Describe broadcast and unicast addressing
Describe relay agent communications
Discuss Microsoft DHCP scopes and classes
Use DHCP troubleshooting utilities
Introducing DHCP : IP Addressing and Related Topics 4 Introducing DHCP DHCP
Service that provides a way for client computer to request IP address assignment
Delivers necessary configuration information to clients
Traces its origins back to an earlier protocol named BOOTP
Servers can manage one or more ranges of IP addresses
How DHCP Works : IP Addressing and Related Topics 5 How DHCP Works When TCP/IP is configured on client computer
The Obtain an IP address automatically option is the only necessary set-up element
Next time workstation attempts to access network
It broadcasts a DHCP address request to the network because it has no IP address
All DHCP servers present on same broadcast domain
Receive request and send back message that indicates a willingness to grant an address lease
How DHCP Works (continued) : IP Addressing and Related Topics 6 How DHCP Works (continued) Client
Accepts address lease offer and sends packet to server that extended offer
In reply
Server proffers an IP address for a specific period of time that the client uses thereafter
When half the lease period expires
Client attempts to renew the lease
Slide 7 : IP Addressing and Related Topics 7
The Role of Leases : IP Addressing and Related Topics 8 The Role of Leases Leases
“Loans” of an address for a specific amount of time
Leases that range from one to three weeks
Typical on networks in which machines seldom move and the workforce is stable
Lease periods
Average between one and three days for networks on which roving workers come and go regularly
Lease periods of four to eight hours
Common on ISP networks
DHCP Software Elements : IP Addressing and Related Topics 9 DHCP Software Elements DHCP client
Enabled at client machine when you select the Obtain an IP address automatically option
DHCP server
Manages address pools and configuration data
DHCP relay agent
Intercepts address requests on local cable segment
Repackages requests as a unicast to one or more DHCP servers
DHCP Lease Types : IP Addressing and Related Topics 10 DHCP Lease Types Manual address lease
Administrator assigns an IP address manually
DHCP server
Assigns addresses for specific periods of time
More About DHCP Leases : IP Addressing and Related Topics 11 More About DHCP Leases How DHCP integrates with DNS
Server addresses are advertised using DNS
All address updates must be entered manually
Client addresses usually come into play only when
e-mail addresses of the form user@domain.name must be resolved
Understanding IP Address Management with DHCP : IP Addressing and Related Topics 12 Understanding IP Address Management with DHCP DHCP Discovery
Occurs when DHCP client broadcasts a request for an IP address
DHCP Discovery
Relies on an initial DHCP broadcast
DHCP servers
PING an address before offering it to the client
Windows clustering
Allows two or more servers to be managed as a single system
DHCP Address Discovery : IP Addressing and Related Topics 13 DHCP Address Discovery Uses four packets
DHCP Discover packet
DHCP Offer packet
DHCP Request packet
DHCP Acknowledgment packet
Slide 14 : IP Addressing and Related Topics 14
The Discover Packet : IP Addressing and Related Topics 15 The Discover Packet During DHCP Discovery process
Client broadcasts Discover packet that identifies client’s hardware address
Header contains
Source IP address 0.0.0.0
Preferred address
Typically the last address the client used
The Offer Packet : IP Addressing and Related Topics 16 The Offer Packet Send by DHCP server to offer IP address to DHCP client
Offer packet
Includes IP address that is offered to the client and,
Sometimes, answers to the requested options in the DHCP Discover packet
Slide 17 : IP Addressing and Related Topics 17
The Request Packet : IP Addressing and Related Topics 18 The Request Packet Once Offer packet is received
Client can either accept offer by issuing a DHCP Request packet, or
Reject offer by sending a DHCP Decline packet
Typically
Client sends a Decline only if it receives more than one Offer
The Acknowledgment Packet : IP Addressing and Related Topics 19 The Acknowledgment Packet Sent from server to client to indicate the completion of the four-packet DHCP Discovery process
Response
Contains answers to any configuration options requested by client in the previous Request packet
The Address Renewal Process : IP Addressing and Related Topics 20 The Address Renewal Process When DHCP client receives an address from a DHCP server
Client also receives a lease time and notes the time that the address was received
Lease time
Defines how long the client can keep the address
In middle of the lease period
Client starts a renewal process
The Renewal Time (T1) : IP Addressing and Related Topics 21 The Renewal Time (T1) T1
Defined as the time that the client tries to renew its network address
The Renewal packet
Is unicast directly to the DHCP server
Default value for T1
0.5 * duration_of_lease (i.e., lease time)
The Rebinding Time (T2) : IP Addressing and Related Topics 22 The Rebinding Time (T2) The time that client begins to
Broadcast a renewal request for an extended lease time from another DHCP server
DHCP specification, RFC 2131, defines default value for T2 as
0.875 * duration_of_lease
Slide 23 : IP Addressing and Related Topics 23
Slide 24 : IP Addressing and Related Topics 24
The DHCP Address Release Process : IP Addressing and Related Topics 25 The DHCP Address Release Process Client should release its address by
Sending a DHCP Release packet to the server
DHCP Release packet
Sent over UDP
DHCP server does not send any acknowledgment
If client does not send the DHCP Release packet
DHCP server automatically releases the address at the lease expiration time
DHCP Packet Structures : IP Addressing and Related Topics 26 DHCP Packet Structures Fields
Operation Code (OPCODE) or Message Type Field
Hardware Type Field
Hardware Length Field
Hops Field
Transaction ID Number Field
Seconds Since Boot or Seconds Elapsed Field
Flags Field
DHCP Packet Structures (continued) : IP Addressing and Related Topics 27 DHCP Packet Structures (continued) Fields
Client IP Address Field
Your IP Address Field
Server IP Address Field
Gateway or Relay Agent IP Address Field
Client Hardware Address Field
Server Host Name Field
Boot File Field
Slide 28 : IP Addressing and Related Topics 28
Slide 29 : IP Addressing and Related Topics 29
DHCP Option 53: Message Type : IP Addressing and Related Topics 30 DHCP Option 53: Message Type Only DHCP option required in all DHCP packets
Indicates general purpose of any DHCP message
DHCP boot sequence uses these message types:
DHCP MessageType 1: Discover (client to server)
DHCP MessageType 2: Offer (server to client)
DHCP MessageType 3: Request (client to server)
DHCP MessageType 5: ACK (server to client)
Slide 31 : IP Addressing and Related Topics 31
Broadcast and UNICAST in DHCP : IP Addressing and Related Topics 32 Broadcast and UNICAST in DHCP DHCP clients
Must broadcast service requests until they obtain IP addresses
DHCP clients
Use unicast addressing after they obtain an address for a local DHCP server or relay agent
DHCP servers
Examine DHCP packets coming from clients
Slide 33 : IP Addressing and Related Topics 33
Communications with a DHCP Relay Agent : IP Addressing and Related Topics 34 Communications with a DHCP Relay Agent DHCP boot up process r
Relies heavily on broadcasts, but most routers do not forward broadcasts
Relay agent function
Typically loaded on a router connected to the segment containing DHCP clients
Relay agent device
Configured with the address of the DHCP server
Can communicate using unicast packets directly with that server
Slide 35 : IP Addressing and Related Topics 35
Slide 36 : IP Addressing and Related Topics 36
Microsoft DHCP Scopes and Classes : IP Addressing and Related Topics 37 Microsoft DHCP Scopes and Classes Address scopes
Define a set of addresses that a DHCP server can assign to clients
Superscopes
A collection of scopes that contain sets of non-consecutive IP addresses
Setting up a Simple DHCP Server : IP Addressing and Related Topics 38 Setting up a Simple DHCP Server Field values in the Dynamic Pool Settings
IP Assigned From
The Range of Pool (1-255):
SubMask
Gateway
Wins
Setting up a Simple DHCP Server (continued) : IP Addressing and Related Topics 39 Setting up a Simple DHCP Server (continued) Field values in the Dynamic Pool Settings
DNS
Domain Name
Lease Time (60 – 31536000 sec)
Status
Slide 40 : IP Addressing and Related Topics 40
Troubleshooting DHCP : IP Addressing and Related Topics 41 Troubleshooting DHCP One good way to troubleshoot DHCP
Use a protocol analyzer, such as Ethereal
Analyzer
Can display the sequence of messages that occurs on a network
Commands to release and/or renew an IP address
ipconfig /release
ipconfig /renew
Slide 42 : IP Addressing and Related Topics 42
Summary : IP Addressing and Related Topics 43 Summary DHCP
Provides a way for computers to obtain usable, unique IP addresses and necessary TCP/IP configurations
From the administrative side
DHCP makes it easy to define and manage pools of IP addresses
DHCP’s origins
Lie in an earlier TCP/IP Application layer protocol, called BOOTP
Summary (continued) : IP Addressing and Related Topics 44 Summary (continued) DHCP
Supports manual or static address allocation
When a DHCP client starts
It begins the process of DHCP Discovery, during which the client receives an IP address and lease
DHCP
Supports a variety of message types and options
Only MessageType 53 (DHCP Message) is mandatory for any given DHCP message
Summary (continued) : IP Addressing and Related Topics 45 Summary (continued) Because DHCP can ferry a surprisingly large range of configuration information
The protocol makes use of several message options
A protocol analyzer is especially effective when diagnosing DHCP difficulties